Non-woven webs have been manufactured for many years for many end uses, including filtration media. Such structures made from bicomponent or sheath-core materials are disclosed in, for example, Wincklhofer et al., U.S. Pat. No. 3,616,160; Sanders, U.S. Pat. No. 3,639,195; Perrotta, U.S. Pat. No. 4,210,540; Gessner, U.S. Pat. No. 5,108,827; Nielsen et al., U.S. Pat. No. 5,167,764; Nielsen et al., U.S. Pat. No. 5,167,765; Powers et al., U.S. Pat. No. 5,580,459; Berger, U.S. Pat. No. 5,620,641; Berger, U.S. Pat. No. 6,174,603; Hollingsworth et al., U.S. Pat. No. 6,146,436; Dong, U.S. Pat. No. 6,251,224; Sovari et al., U.S. Pat. No. 6,355,079; Hunter, U.S. Pat. No. 6,419,721; Cox et al., U.S. Pat. No. 6,419,839; Stokes et al., U.S. Pat. No. 6,528,439; Amsler, U.S. Pat. No. H2,086 and Amsler, U.S. Pat. No. 6,267,252. Such structures have been made by both air laid and wet laid processing and have been used in both air and liquid filtration applications with some degree of success. In this regard we have found that the previous non-woven webs that are used for particulate removal from mobile fluids often suffer from a lack of mechanical strength and short useful life when wet by liquid under filtration condition. Often the pressure of the fluid exceeds the wet burst strength of the media. The media can also become rapidly plugged and reduced in permeability causing reduced flow or failure of fluid movement. Burst or plugged media rapidly fail to filter particulate. A burst media would fail to protect the equipment from particulate it was designed to protect. Substantially increased or high pressure drop would restrict the fluid flow resulting in loss of fluid flow and power.
Many attempts to obtain non-woven structures with suitable perforate or porous supports have been attempted. Melt blown materials and thermal lamination techniques have been attempted, however, the resulting structures often obtain incorrect pore sizes, reduced efficiency, reduced permeability, lack of strength or other problems rendering the media or filter structure insufficient for useful filtration applications.
A substantial need exists for filtration media, filter structures and filtration methods that can be used for removing particulate materials from liquid compositions, such as aqueous liquids and in particular, non-aqueous liquids such as fuels, lubricating oils and hydraulic fluids. The invention provides such media, filtration structures and methods and provides a unique media or media layer combinations that achieve substantial permeability, high wet strength, substantial efficiency and long filtration life.